1. Field of the Invention
The present invention relates generally to wireless communication systems and more particularly to a method and apparatus for dynamically allocating identification address information to a communication device, such as a mobile communication terminal in a satellite communication system.
2. Description of the Related Art
The use of wireless communication systems is well known for transmitting information between fixed stations and one or more geographically dispersed mobile receivers. For example, satellite communication systems have been used in the trucking industry for many years to provide messaging and location information between fleet-owned dispatch centers and their respective tractor-trailer vehicles. Such systems offer significant benefits to fleet owners because they allow almost instantaneous communications and real-time position information. In addition, many such systems provide remote monitoring of the performance characteristics of each fleet-owned vehicle, such as average speed, RPM, idle time, and so on. An example of such a satellite communication system is disclosed in U.S. Pat. No. 4,979,170 entitled "ALTERNATING SEQUENTIAL HALF DUPLEX COMMUNICATION SYSTEM AND METHOD", U.S. Pat. No. 4,928,274 entitled "MULTIPLEXED ADDRESS CONTROL IN A TDM COMMUNICATION SYSTEM", and U.S. Pat. No. 5,017,926 entitled "DUAL SATELLITE NAVIGATION SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
In the satellite communication system described by the abovementioned patents, fleet-owned dispatch centers communicate by way of land-based systems such as telephone or fiber-optic systems to a hub, otherwise known as a network management facility (NMF). The NMF acts as a central communication station through which all communications between vehicles and dispatch centers pass. The NMF comprises a number of network management computers (NMCs), each NMC being responsible for providing a communication path from the NMF to geographically dispersed vehicles in the communication system by way of a geosynchronous satellite. The geosynchronous satellite comprises one or more transponders, which are electronic circuits well known in the art for relaying high frequency satellite communication signals between remote locations. Each NMC is assigned an individual transponder, each transponder operating at a unique frequency in order to avoid interference with communication signals on other transponders. In the satellite communication system of the above-referenced patents, each transponder is capable of handling the communications of approximately 30,000 vehicles.
Each vehicle in the communication system is equipped with a transceiver, otherwise known as a mobile communication terminal (MCT), for communicating message and location information to a pre-designated NMC by way of the geosynchronous satellite. The MCT typically also comprises an interface device which displays messages to one or more vehicle occupants, and accepts either voice or text messages to be transmitted to a dispatch center corresponding to the particular vehicle. Furthermore, the MCT may further comprise a digital computer which communicates with one or more Electronic Control Units (ECUs) located at various points throughout the vehicle. Each ECU provides operational information to the digital computer indicating performance characteristics such as vehicle speed, engine RPM, and miles traveled, and other operating characteristics.
NMCs and MCTs communicate with each other using a pre-defined messaging protocol. The messaging protocol arranges digital data into manageable segments, called frames, which include address frames and data frames. Typically, an address frame is transmitted followed by one or more data frames. The address frame contains the necessary MCT identification information in order to route messages from an NMC to one or more identified vehicles and to identify the origin of incoming messages from vehicles. Data frames contain the actual information which is to be conveyed, for example, voice, operation, or location data.
An important feature of the satellite communication system just described is the ability for NMCs to uniquely identify MCTs which are operating in the communication system. The ability to uniquely identify MCTs allows NMCs to route messages to individual vehicles for which they are intended and also to identify the origin of incoming messages from vehicles. In the satellite communication system described by the above-mentioned patents, each MCT in the system is permanently assigned a unique identification address from a set of available identification addresses stored within the NMF. Each assigned identification address identifies a particular MCT, the address being identical to a unique, pre-assigned MCT serial number, assigned at a manufacturing facility. The identification address is attached to each message transmitted, using the address frame, to identify the intended recipient or the source of messages. The number of MCTs which can be uniquely identified in the system is limited by the maximum size of the address frame. In the communication system described in the above-referenced patents, the address field in the address frame is 19 bits in size. Therefore, the maximum number of MCTs which can be uniquely identified in the system is 2.sup.19, or 524,288 MCTs.
The maximum number of MCTs that can be uniquely identified in current communication systems using the fixed address frame protocol is approaching rapidly, as new MCTs are added to the system on a daily basis. As such, there is a need to modify the existing communication protocol in order to allow a greater number of MCTs to operate in the system over and above the maximum limitation set by the address frame. Simply increasing the size of the address frame is not a feasible solution, as this would require a complete overhaul of the message protocol and a recall of all MCTs currently in use. Nor would the creation of a completely new communication system be acceptable, as this would also require major hardware and software modifications to existing MCTs and NMFs. Therefore, a modification of the present system is desirable which should be backward-compatible with existing MCTs to eliminate the need for a recall of the existing units.